As a superconducting cable, the superconducting cable 100 shown in FIG. 8 is proposed, which is structured such that three cable cores 10 are housed in a thermal insulation pipe 20 (See, for example, Patent documents 1 and 2).
The cable core 10 includes a former 11, a conductor layer 13, an electrical insulation layer 16A, a shielding layer (outer-conductor layer) 17A, and a protective layer 18 in the enumerated order from the center. The conductor layer 13 is formed by spirally winding superconducting wires in multiple layers around the former 11. Generally, the superconducting wire has a tape-shaped structure in which a plurality of filaments consisting of oxide superconducting material are arranged in a matrix of a silver sheath or the like. The electrical insulation layer 16A is formed by winding an insulation paper. The shielding layer 17A is formed by spirally winding a superconducting wire around the electrical insulation layer 16A as in the case of the conductor layer 13. An insulation paper or the like is used for the protective layer 18.
The thermal insulation pipe 20 is structured such that a thermal insulation material (not illustrated) is arranged between the double pipes consisting of an inner pipe 21 and an outer pipe 22 and the space between the double pipes is evacuated. An anticorrosion layer 23 is formed around the outside of the thermal insulation pipe 20. The thermal insulation pipe 20 is prepared for a usable condition by filling and circulating a coolant of liquid-nitrogen or the like in the spaces formed inside the former 11 (in the case where it has a hollow structure) as well as between the inner pipe 21 and the cores 10 such that the electrical insulation layer 13 is impregnated with the coolant.
The structure of such a superconducting cable is studied mainly as an alternating current (AC) cable. In such case, it is proposed that for the purpose of reducing AC loss, the winding pitch and winding direction of a superconducting wire in each layer be individually adjusted in order to enable an equivalent current to flow through the respective layers which constitute a conductor layer and a shielding layer. Also, for the purpose of reducing the amount of superconducting wires to be used, generally a comparatively large value is chosen as a winding pitch of the superconducting wires.
[Patent document 1] Japanese Patent Application Publication No. 2003-249130 (FIG. 1)
[Patent document 2] Japanese Patent Application Publication No. 2002-140944 (FIG. 2)